The invention relates generally to line drivers, and more specifically to an arrangement for reducing power dissipation in line drivers connected to transmission lines in multitone systems, such as e.g., ADSL (Asymmetric Digital Subscriber Line) systems.
In a multitone system, such as an ADSL-system, data information is randomly coded into the phase and amplitude of a plurality of sine tones that are transmitted by line drivers on transmission lines in bursts with a so-called symbol rate or length, which in the ADSL case is 246.3 xcexcs. To enable Fourier transformations to be made on each symbol, a so-called cyclic prefix is inserted between the symbols. The cyclic prefix does not contain any valid data but ensures that the multitone signal appears continuous for the Fourier transformation. In the ADSL case, each cyclic prefix has a length of 32 samples or about 15 xcexcs.
Since the phase and amplitude of each individual tone can be seen as random and the total signal consists of a sum of many tones, there will be a great difference between successive symbols in the peak-to-peak voltage of the total signal that is to be transmitted. In typical ADSL systems, the supply voltage of the line drivers that are connected to the transmission line for transmitting multitone signals must be selected such that the theoretical maximum peak can be transmitted without being clipped. But this causes the supply voltage of the line driver to be far too high for the majority of the symbols that are transmitted. Consequently, the power dissipation in the line driver will be higher than necessary for most of the symbols.
An object of the invention is to reduce the power dissipation in such line drivers. The supply voltage to the line driver is controlled from symbol-to-symbol during the cyclic prefixes in response to a known, expected, peak-to-peak value of the next symbol to be transmitted. By adapting the supply voltage to expected peak-to-peak values, the power dissipation is reduced in the line driver.